int expire_day = expire_date.Day;// expire_date is the date fetched from database
DateTime today = DateTime.Now;
int now = today.Day;
lblMessage.Text = (expire_day - now).ToString() + " remains!!!";

<form id="form1" runat="server">
<asp:ScriptManager ID="ScriptManager1" runat="server" />

<asp:Timer runat="server" id="Timer1" interval="5000" ontick="Timer1_Tick" />
<asp:UpdatePanel runat="server" id="TimePanel" updatemode="Conditional">
<Triggers>
<asp:AsyncPostBackTrigger controlid="Timer1" eventname="Tick" />
</Triggers>
<ContentTemplate>
<asp:Label runat="server" id="Label1" />
</ContentTemplate>
</asp:UpdatePanel>
</form>

public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{

}

protected void Timer1_Tick(object sender, EventArgs e)
{
     Label1.Text = DateTime.Now.ToString();
}
}